This invention pertains to the manufacture of photovoltaic cells and more particularly to an improved low-cost method of fabricating polycrystalline silicon solar cells wherein the damaged surface layer generated during hydrogen passivation is used as a plating mask for the metallization of the front surface electrodes.
Heretofore a common method of fabricating silicon solar cells has included the steps of forming a PN junction by diffusing a suitable dopant into the front side of a silicon wafer or ribbon, etching a grid electrode pattern in a protective dielectric masking layer formed on that front surface, depositing a nickel plating on all silicon exposed by the etching, overplating the nickel with copper and tin, removing the remainder of the dielectric masking layer from the front surface, and providing an anti-reflection coating on the newly exposed portions of the front surface.
While such a procedure may be applied to either single crystal or polycrystalline silicon, cost considerations make it desirable to fabricate solar cells from the latter. However, as is well known, because of the minority carrier losses at grain boundaries, dislocations, and the like, the efficiencies achieved with polycrystalline silicon solar cells are generally poorer than those of monocrystalline cells. This circumstance has been improved upon by introducing a monovalent element, such as hydrogen, into the structure to combine with the dangling bonds associated with the structural defects, thereby minimizing the minority carrier recombination loss.
As is known in the art, an important consideration in designing a cell processing sequence is that the combination of time and temperature in any step following the hydrogen passivation step should not cause the hydrogen introduced into the silicon to be diffused back out of the passivated substrate. Thus, for instance, it has been found that a hydrogen passivated cell subjected to a temperature of 600.degree. C. for one-half hour in a vacuum loses substantially all the bonded hydrogen and returns to its pre-passivation level, as evidenced by its observed electron beam induced current activity. It should be noted in this regard that the function diffusion step in solar cell fabrication typically involves temperatures on the order of 900.degree. C.
It has also been found that hydrogen passivation normally heats the cell to a high enough temperature to cause base metals, such as copper, to migrate through the junction, thereby causing a "soft" diode or a short circuit. As shown, for instance, by C. H. Seager, D. J. Sharp, J. K. G. Panitz, and R. V. D'Aiello in Journal of Vacuum Science and Technology, Vol. 20, no. 3, pp 430-435 (March 1982), passivation of polycrystalline silicon may be accomplished with a Kaufman-type ion source used to produce a hydrogen ion beam in the kilo electron volt energy range. Relatively short exposure times (e.g. between 0.5 and 4 minutes) in a high ion energy and flux (e.g. 1 to 3 milliamperes per square centimeter) range appear to be optimal. Such exposures generally result in the substrate temperature rising to at least approximately 275.degree. C., if the substrate is carefully contacted to an appropriate heat sink. Otherwise, temperatures in excess of 400.degree. C. are readily achieved. It is important, however, that temperatures be limited to less than about 300.degree. C. to avoid rapid migration of base metals into the silicon matrix. However, manipulation of substrate and heat sink to effect thermal control during passivation easily becomes the rate limiting factor in high throughput processing with such ion sources. Consequently, it is desirable to avoid heat sinking in order to obtain a low cost, high throughput process. Additionally, for EFG-type silicon ribbon, which may be economically produced, surface irregularities make heat sinking difficult.
Additionally, hydrogen passivation is most effective when the base silicon surface is exposed. Thus, any "positive" plating mask used to define the front surface grid electrode pattern by covering the interelectrode area of the front surface should not be in place during passivation.
As described in copending application Ser. No. 563,061, the altered surface layer produced in hydrogen ion beam passivation may be used as a plating mask for subsequent metallization steps involving immersion plating of a selected metal. A preferred embodiment of the process described in detail in application Ser. No. 563,061 as applied to the manufacture of silicon solar cells involves, inter alia, the following steps: (1) forming a plating mask of a dielectric material on the front surface of a shallow-junction silicon ribbon so as to leave exposed those areas of the silicon to be later covered by the front surface electrode, (2) depositing a thin layer of nickel (or similar material) on the exposed silicon, (3) removing the plating mask, (4) hydrogen passivating the junction side of the cell, (5) sintering the nickel to form in part a nickel silicide, (6) immersion plating additional nickel onto the metal-covered portions of the cell, (7) electroplating a layer of copper onto the nickel, and (8) applying an anti-reflection coating over the exposed surface of the silicon. Thereafter, the silicon may be further processed, e.g. to prepare it for connection to electrical circuits. In an alternative process, the heating of the sample during passivation supplies at least part of the energy for the nickel sintering step.
While it will be appreciated that such a procedure (1) permits the removal of the initial plating mask prior to passivation (permitting better passivation) and (2) allows passivation prior to the application of base metals without the requirement of an additional masking step prior to metallization (both eliminating the danger of spoiled cells caused by migration of the base metal during passivation and simplifying the production process by eliminating the need either for close thermal control of the substrate during passivation or for a photolithographic step following passivation), the process may still be improved. Thus, although the thermal control required to prevent base metal migration (temperatures preferably less than about 300.degree. C.) is obviated, there is still the danger of spoiled cells due to migration, albeit at a slower diffusion rate, of nickel or nickel silicide within the matrix of the substrate.
Further, the method just outlined requires the formation of a plating mask, as an additional layer on the substrate, prior to the initial metallization, and to this extent requires additional processing and materials.